Method for producing a hot-formed steel component, and hot formed steel component

ABSTRACT

The invention relates to a method for producing a component by hot-forming a pre-product composed of steel, wherein the pre-product is heated to a temperature above 60° C. and below the Ac3 transformation temperature and then formed in this temperature range, wherein the component has a minimum tensile strength of 700 MPa and high elongation at break, wherein the pre-product has the following alloy composition in percent by weight: C: 0.0005 to 0.9; Mn: more than 3.0 to 12; the remainder iron including unavoidable steel-accompanying elements, with the optional addition of one or more of the following elements (in percent by weight): Al: up to 10; Si: up to 6; Cr: up to 6; Nb: up to 1.5; V; up to 1.5; Ti: up to 1.5; Mo: up to 3; Cu: up to 3; Sn: up to 0.5; W up to 5; Co: up to 8; Zr: up to 0.5; Ta: up to 0.5; Te: up to 0.5; B: up to 0.15; P: at most 0.1, in particular &lt;0.04; S: at most 0.1, in particular &lt;0.02; N: at most 0.1, in particular &lt;0.05; Ca: up to 0.1. The invention further relates to a hot-formed component produced from a steel.

The invention relates to a method for producing a component byhot-forming a pre-product of steel. Pre-products for hot-forming areunderstood hereinafter to be e.g. sheets cut from the coil or plateblanks or seamless or welded pipes which occasionally can additionallybe cold-drawn. The invention also relates to a hot-formed componentproduced from a steel.

Such components produced by hot-forming are used mainly in theautomotive and commercial vehicle industries, but there are alsopossible applications in engineering for producing white goods or in theconstruction industry.

The fiercely competitive market means that automobile producers areconstantly forced to find solutions for reducing fleet consumptionwhilst maintaining the highest possible level of comfort and occupantprotection. On the one hand, the weight saving of all of the vehiclecomponents plays a decisive role as does, on the other hand, the mostfavourable possible behaviour of the individual components in the eventof high static and dynamic loading during operation and also in theevent of a crash.

Furthermore, the reduction in CO₂ emissions along the entiremanufacturing chain represents a particular challenge which is met byinnovative solutions in terms of process technology. In particular, thefocal point is on process steps which are based directly or indirectlyupon the combustion of fossil fuels.

The suppliers of pre-materials attempt to take this requirement intoconsideration in that by providing high-strength and super high-strengthsteels the wall thicknesses can be reduced whilst at the same timeachieving improved component behaviour during manufacture and operation.

Therefore, these steels must satisfy comparatively stringentrequirements in terms of strength, extensibility, toughness, energyconsumption and corrosion-resistance and their processability e.g.during cold-forming and welding.

Amongst the aforementioned aspects, the production of components ofhot-formable steels is acquiring increasing significance because theyare ideal for meeting the increased requirements upon componentproperties, with material outlay being reduced.

The production of components by means of quenching of pre-products ofpress-hardenable steels by hot-forming in a forming tool is known frompatent document IDE 601 19 826 T2. In this case, a sheet platepreviously heated above the austenitization temperature to 800-1200° C.and possibly provided with a metallic coat of zinc or on the basis ofzinc is formed in an occasionally cooled tool by hot-forming to producea component, wherein during forming or after hot-forming, by reason ofrapid heat extraction the sheet or component in the forming toolundergoes quench-hardening (press-hardening) and thereby obtains therequired microstructure properties and strength properties.

The metallic coat is applied as corrosion protection, typically in thecontinuous hot-dipping method, onto a hot strip or cold strip or ontothe pre-product produced therefrom, e.g. as hot-dip galvanising oraluminium coating.

Subsequently, the plate is suitably cut to size for the forming tool ofthe hot-forming procedure It is also possible to provide the workpieceto be formed in each case, or the blank, with a hot-dip coat.

The application of a metallic coat onto the pre-product, to be formed,prior to hot-forming is advantageous in the case of this method becauseduring press-hardening a disadvantageous change in the surface of thesteel substrate caused by scaling of the basic material can beeffectively avoided by reason of the coat and excessive tool wear can beeffectively avoided by reason of an additional lubricating effect.

Known steels for this application which are suitable for press-hardeningare e.g. manganese-boron steel “22MnB5”.

In order to obtain components having very high strengths of more than980 MPa whilst maintaining a sufficiently high level of toughness, it isknown from laid-open document EP 2 546 375 A1 to correspondingly form asteel having a microstructure, which is predominantly ferritic in theinitial state and has perlite proportions, by means of press-formhardening, and to adjust, by means of stepwise process control, amicrostructure of bainite, tempered martensite and residival austeniteon the finished component. In this case, the sheet to be formed isheated initially to a temperature of 750 to 1000° C. and is maintainedat this temperature for 5 to 1000 seconds, then said sheet is formed at350 to 900° C. and cooled to 50 to 350° C. Finally, said sheet isreheated to a temperature of 350 to 490° C. and this temperature ismaintained for a period of 5 to 1000 seconds. The microstructure on thefinished component has 10 to 85% martensite, 5 to 40% residual austeniteand at least 5% bainite.

However, the production of a component by hot-forming by means ofpress-hardening has several disadvantages.

On the one hand, this method requires a large amount of energy onaccount of the heating of the pre-product to austenitization temperatureper se and additionally for the conversion of ferrite into austenite,which makes the method expensive and results in considerable CO₂emissions.

Moreover, in order to avoid excessive scaling of the sheet surface, asdescribed above, an additional metallic protective layer or anadditional lacquer-based protective layer is required or considerableamount of reworking of the surface which has become scaled by heatingand forming is required.

Moreover, since forming is performed at temperatures above the Ac₃temperature, in general significantly above 800° C., extremely stringentrequirements in terms of temperature stability are applied to theseprotective layers and therefore cathodic corrosion protection of thepre-product on a basis of zinc can be used in this case only to alimited extent and with increased process outlay because the zincvaporises at these high temperatures. Consequently, the process ofpress-hardening uses predominantly steel sheets which have an AlSicoating but which do not offer any cathodic corrosion protection of theformed component.

A further disadvantage is that the formed component must be cooled in anaccelerated manner in the forming tool itself, a further tool outsidethe forming press or using gaseous or liquid media in order to achievethe desired level of strength. The duration of this cooling procedureconsiderably reduces the throughput of components per unit of time, thusreducing economic feasibility.

In summary, it can be stated that the known method for producingcomponents of steel by hot-forming by means of press-hardening above theaustenitization temperature Ac₃ results in high manufacturing and energycosts and therefore high component costs on account of the requiredlarge heating furnaces associated with long heating times and thecooling of the component in the tool as required at the end of theprocess. Moreover, it is not possible to ensure any cathodic corrosionprotection by the application of a coating prior to heating and forming.

Laid-open document DE 10 2011 108 162 A1 discloses a method forproducing a component by semi-hot-forming a pre-product of steel belowthe Ac₁ conversion temperature, in which the required increase instrength in the component is achieved by cold-forming the pre-productprior to heating to forming temperature. Optionally, an additionalincrease in strength in the component can be achieved by usinghigher-strength materials, such as bainitic, martensitic, micro-alloyedand dual-phase or multi-phase steels. The disadvantage in this case isthe additional outlay caused by the necessary cold-forming prior toheating to forming temperature. During hot-forming, dual-phase steelsalso have the disadvantage of sensitivity to edge crack-induced failureduring forming. References to alloy compositions to be specificallyobserved or specifications for the microstructure of the pre-product forspecific adjustment of the mechanical properties of the component aftersemi-hot-forming when using higher-strength steels are not disclosed.

Laid-open document DE 10 2013 009 232 A1 discloses a method forproducing a component by semi-hot-forming a pre-product of steel, inwhich the pre-product is heated to forming temperature and is thenformed, wherein, after forming, the component has a bainiticmicrostructure having a minimum tensile strength of 800 MPa. Heating isperformed to a temperature below the Ac₁ conversion temperature, whereinthe pre-product already is made of a steel having a microstructure of atleast 50% bainite, and wherein the pre-product has the following alloycomposition in wt.%: C: 0.02 to 0.3; Si: 0.01 to 0.5; Mn: 1.0 to 3.0; P:max. 0.02; S: max. 0.01; N: max. 0.01; Al: to 0.1; Cu: to 0.2; Cr: to3.0; Ni: to 0.2; Mo: to 0.2; Ti: to 0.2; V: to 0.; Nb: to 0.1 and B: to0.01.

Although this alloying concept can already be used to produce componentshaving a very high tensile strength of over 800 MPa and an expansion ofover 10% and to produce cathodic corrosion protection of zinc, theforming capability of this material still does not meet the moststringent requirements for the production of complex componentgeometries. In particular, the elongation at fracture achieved and thestrength are still too low for many requirements.

The object of the invention is to provide a method for producing acomponent by hot-forming a pre-product of steel at temperatures belowthe Ac₃ conversion temperature, which is cost-effective and by means ofwhich a once again improved forming capability of the steel material isachieved in the component with minimum tensile strengths of 700 MPa tobe achieved. A corresponding component which is produced bysemi-hot-forming is also to be provided.

A method in accordance with the invention is described in claims 1 to 33and a hot-formed component in accordance with the invention is describedin claims 34 to 38.

According to the teaching of the invention, this object is achieved by amethod for producing a component by hot-forming a pre-product of steel,in which the pre-product is heated to a temperature above 60° C. andbelow the Ac₃ conversion temperature and is then formed, wherein thecomponent has a minimum tensile strength of 700 MPa with an elongationat fracture of over 22%, wherein the pre-product has the following alloycomposition in wt. %:

C: 0.0005 to 0.9

Mn: more than 3.0 to 12

with the remainder being iron including unavoidable steel-associatedelements, with optional addition by alloying of one or more of thefollowing elements (in wt. %): Al: to 10, Si: to 6, Cr: to 6, Nb: to1.5, V: to 1.5, Ti: to 1.5, Mo: to 3, Cu: to 3, Sn: to 0.5, W to 5, Co:to 8, Zr: to 0.5, Ta: to 0.5, Te: to 0.5, B: to 0.15, P: max. 0.1, S:max. 0.1, N: max. 0.1, Ca to 0.1.

The steel used for the method in accordance with the invention has amulti-phase microstructure, comprised of ferrite and/or martensiteand/or bainite and residual austenite. The residual austenite proportioncan be 5% to 80%. The residual austenite can partially or completelyconvert into martensite by the TRIP effect when mechanical stresses arepresent. The alloy in accordance with the invention has a TRIP and/orTWIP effect when subjected to mechanical stress accordingly. Owing tothe strong solidification (similar to cold solidification) induced bythe TRIP and/or TWIP effect and by the increase in the dislocationdensity, the steel achieves very high values in terms of elongation atfracture, in particular uniform elongation, and tensile strength. In anadvantageous manner, this property is achieved by the residual austenitepresent only with manganese contents of over 3 wt. %.

The use of the term “to” in the definition of the content ranges, suchas e.g. 0.01 to 1 wt. %, means that the limit values −0.01 and 1 in theexample—are also included.

The steel in accordance with the invention is suitable in particular forproducing complexly formed components by semi-hot-forming which have notonly a very good forming capability during forming but also have highstrength and elongation at fracture in the operating state andadvantageously are provided with zinc-based cathodic corrosionprotection.

Advantageously, the steel in the initial state has a tensile strength Rmof >700 to 2000 MPa with an elongation at fracture A₈₀ in dependenceupon the achieved tensile strength of at least 3 to 40%.

In contrast to the method for producing a component by means ofpress-hardening which is known from DE 601 19 826 T2 or EP 2 546 375 A1,the method in accordance with the invention has the advantage that witha considerably lower energy requirement for the heating procedure theuse of a multi-phase steel in the initial state having residualaustenite serves to provide a component having mechanical characteristicvalues of tensile strength and elongation at fracture which areconsiderably better than the mechanical properties of the components ofknown steels for semi-hot-forming. In addition, in comparison withpress-hardening, energy costs are saved by reason of lower heatingtemperatures.

The steel containing medium manganese and comprising a manganese contentof over 3 wt. % is provided as a flat product (hot strip or cold strip)or as a seamless or welded pipe having a corrosion protection layer (Zn,Zn alloys, inorganic or organic coatings with Zn, AlSi or otherinorganic or organic coatings) and is subsequently warm-formed (HWU).Warm is defined in this case as forming after heating of the pre-productto a temperature <700° C., preferably <450° C., more preferably <350° C.to 60° C., wherein the austenite proportion in the pre-product isretained completely or partially during forming and the possible onsetof a TRIP effect is suppressed completely or partially. Heating to <450°C., preferably <350° C., facilitates the use of cathodic, zinc-basedcorrosion protection.

Furthermore, semi-hot-forming improves the forming properties incomparison with forming at RT and advantageously increases theresistance to hydrogen embrittlement and delayed crack formation.Cooling is performed in still air, i.e. compared to press-hardening itdoes not require any accelerated and/or regulated cooling.

The component can be cooled technically in an accelerated manner,optionally after semi-hot-forming, by means of airflow, oil, water orother active media.

The pre-product and the hot-formed component manufactured therefromhave, before and after semi-hot-forming, a tensile strength of 700 to2000 MPa, preferably 850 to 1800 MPa, particularly preferably >1000 to1.800 MPa at expansions A80 of >3 to 40%, preferably >6 to 30%. Higherrequired expansions tend to produce lower strengths and vice versa.

Therefore, the product of required tensile strength and achievedelongation at fracture (Rm×A₈₀) can be considered to be decisive forcharacterising these component properties.

Tests on the finished component, i.e. after hot-forming, advantageouslydemonstrate the following excellent results for the product of Rm×A₈₀ inMPa %:

Rm of 700 to 800 MPa: Rm×A80≥15400 up to 50000

Rm of over 800 to 900 MPa: Rm×A₈₀≥14400 up to 50000

Rm of over 900 to 1100 MPa: Rm×A₈₀≥13500 up to 45000

Rm of over 1100 to 1200 MPa: Rm×A₈₀≥13200 up to 45000

Rm of over 1200 to 1350 MPa: Rm×A₈₀≥11200 up to 45000

Rm of over 1350 to 1800 MPa: Rm×A₈₀≥8000 up to 45000

Rm of over 1800 MPa: Rm×A₈₀≥4000 up to 30000

Heating of the material which is to be warm-formed is preferablyeffected inductively or alternatively by radiation or conductively.Optionally, heating of the material is effected prior tosemi-hot-forming directly in the forming tool, thus making it possibleto save on an additional furnace unit and to omit a process step. Thisis considered in particular at heating temperatures of <450° C.,preferably <350° C.

The customer requirement for cathodic corrosion protection inconjunction with high-strength steels having required tensile strengthsRm of up to 1500 MPa can thus be met in an advantageous manner bysemi-hot-forming of the inventive comprising steels containing more than3 to 12 wt. % manganese. Furthermore, the heating temperature below Ac₃advantageously brings about only a small decrease in the strength of thepre-product whilst at the same time offering improved forming propertiescompared with forming at RT, in which the onset of the TRIP/TWIP effectwould occur. By reason of the fact there is no conversion or else only apartial conversion of the microstructure after semi-hot-forming, thecomponents undergo only slight distortion during cooling.

Furthermore, it is possible to achieve energy saving potentials and areduction in CO₂ emissions during semi-hot-forming compared withpress-hardening at temperatures above Ac₃.

Particularly uniform and homogeneous material properties can be achievedif the steel of the pre-product has the following alloy composition inwt. %:

C: 0.05 to 0.42

Mn: >5 to <10

with the remainder being iron including unavoidable, steel-associatedelements, with optional

addition by alloying of one or more of the following elements (in wt.%):

Al: 0.1 to 5, in particular >0.5 to 3

Si: 0.05 to 3, in particular >0.1 to 1.5

Cr: 0.1 to 4, in particular >0.5 to 2.5

Nb: 0.005 to 0.4, in particular 0.01 to 0.1

B: 0.001 to 0.08, in particular 0.002 to 0.01

Ti: 0.005 to 0.6, in particular 0.01 to 0.3

Mo: 0.005 to 1.5, in particular 0.01 to 0.6

Sn: <0.2, in particular <0.05

Cu: <0.5, in particular <0.1

W: 0.01 to 3, in particular 0.2 to 1.5

Co: 0.01 to 5, in particular 0.3 to 2

Zr: 0.005 to 0.3, in particular 0.01 to 0.2

Ta: 0.005 to 0.3, in particular 0.01 to 0.1

Te: 0.005 to 0.3, in particular 0.01 to 0.1

V: 0.005 to 0.6, in particular 0.01 to 0.3

Ca 0.005 to 0.1

Alloy elements are generally added to the steel in order to influencespecific properties in a targeted manner. An alloy element can therebyinfluence different properties in different steels. The effect andinteraction generally depend greatly upon the quantity, presence offurther alloy elements and the solution state in the material. Thecorrelations are varied and complex. The effect of the alloy elements inthe alloy in accordance with the invention will be discussed in greaterdetail hereinafter. The positive effects of the alloy elements used inaccordance with the invention will be described hereinafter:

Carbon C: is required to form carbides, stabilises the austenite andincreases the strength. Higher contents of C impair the weldingproperties and result in the impairment of the expansion and toughnessproperties, for which reason a maximum content of 0.9 wt. % is set. Theminimum content is set at 0.0005 wt. %. Preferably, a content of 0.05 to0.42 wt. % is set because in this range the ratio of residual austeniteto other phase proportions can be adjusted in a particularlyadvantageous manner.

Manganese Mn: stabilises the austenite, increases the strength and thetoughness and permits a deformation-induced martensite formation and/ortwinning in the alloy in accordance with the invention. Contents of lessthan or equal to 3 wt. % are not sufficient to stabilise the austeniteand thus impair the expansion properties whereas with contents of 12 wt.% and more the austenite is stabilised too much and as a result thestrength properties, in particular the yield strength, are reduced. Forthe manganese steel in accordance with the invention having averagemanganese contents, a range of over 5 to <10 wt. % is preferred becausein this range the ratio of the phase proportions to each other and theconversion mechanisms can be advantageously influenced duringsemi-hot-forming and cold-forming.

Aluminium Al: improves the strength and expansion properties, decreasesthe specific density and influences the conversion behaviour of thealloy in accordance with the invention. Contents of Al of more than 10wt. % impair the expansion properties and cause predominantly brittlefracture behaviour. For the manganese steel in accordance with theinvention having average manganese contents, an Al content of 0.1 to 5wt. % is preferred in order to increase the strength and at the sametime maintain effective expansion. In particular, contents of >0.5 to 3wt. % permit a particularly large product of strength and elongation atfracture.

Silicon Si: impedes the diffusion of carbon, reduces the relativedensity and increases the strength and expansion properties andtoughness properties. Contents of more than 6 wt. % prevent furtherprocessing by cold-rolling by reason of embrittlement of the material.Therefore, a maximum content of 6 wt. % is set. Optionally, a content of0.05 to 3 wt. % is set because contents in this range positivelyinfluence the forming properties. Si contents of >0.1 to 1.5 wt. % haveturned out to be particularly advantageous for forming and conversionproperties.

Chromium Cr: improves the strength and reduces the rate of corrosion,delays the formation of ferrite and perlite and forms carbides. Themaximum content is set to 6 wt. % since higher contents result in animpairment of the expansion properties and substantially higher costs.For the manganese steel in accordance with the invention having averagemanganese contents, a Cr content of 0.1 to 4 wt. % is preferred in orderto reduce the precipitation of coarse Cr carbides. In particular,contents of >0.5 to 2.5 wt. % have proven to be advantageous forstabilising the austenite and precipitating fine Cr carbides.

Molybdenum Mo: acts as a carbide forming agent, increases the strengthand increases the resistance to delayed crack formation and hydrogenembrittlement. Contents of Mo of more than 3 wt. % impair the expansionproperties, for which reason a maximum content of 3 wt. % is set. Forthe manganese steel in accordance with the invention having averagemanganese contents, an Mo content of 0.005 to 1.5 wt. % is preferred inorder to avoid the precipitation of excessively large Mo carbides. Inparticular, contents of 0.01 wt. % to 0.6 wt. % bring about theprecipitation of desired Mo carbides with at the same time reduced alloycosts.

Phosphorus P: is a trace element from the iron ore and is dissolved inthe iron lattice as a substitution atom. Phosphorous increases thehardness by means of mixed crystal solidification and improves thehardenability. However, attempts are generally made to lower thephosphorous content as much as possible because inter alia it exhibits astrong tendency towards segregation owing to its low diffusion rate andgreatly reduces the level of toughness. The attachment of phosphorous tothe grain boundaries can cause cracks along the grain boundaries duringhot-rolling. Moreover, phosphorous increases the transition temperaturefrom tough to brittle behaviour by up to 300° C. For the aforementionedreasons, the phosphorous content is limited to a maximum of 0.1 wt. %,wherein contents <0.04 wt. % are advantageously sought for theaforementioned reasons.

Sulphur S: like phosphorous, is bound as a trace element in the ironore. It is generally not desirable in steel because it exhibits a strongtendency towards segregation and has a greatly embrittling effect,whereby the expansion and toughness properties are impaired. An attemptis therefore made to achieve amounts of sulphur in the melt which are aslow as possible (e.g. by deep vacuum treatment). For the aforementionedreasons, the sulphur content is limited to a maximum of 0.1 wt. %. It isparticularly advantageous to limit the S content to <0.2 wt. % in orderto reduce the precipitation of MnS.

Nitrogen N: N is likewise an associated element from steel production.In the dissolved state, it improves the strength and toughnessproperties in steels containing a high content of manganese of greaterthan or equal to 4 wt. % Mn. Lower Mn-alloyed steels of <4 wt. % withfree nitrogen tend to have a strong ageing effect. The nitrogen evendiffuses at low temperatures to dislocations and blocks the same. Itthus produces an increase in strength associated with a rapid loss oftoughness. Binding of the nitrogen in the form of nitrides is possiblee.g. by alloying aluminium, vanadium, niobium or titanium. For theaforementioned reasons, the nitrogen content is limited to a maximum of0.1 wt. %, wherein contents <0.05 wt. % are preferably sought tosubstantially avoid the formation of AlN.

Microalloy elements are generally added only in very small amounts (<0.1wt. % per element). In contrast to the alloy elements, they mainly actby precipitation formation but can also influence the properties in thedissolved state. Despite the small amounts added, microalloy elementsgreatly influence the production conditions and the processingproperties and final properties.

Typical microalloy elements are vanadium, niobium and titanium. Theseelements can be dissolved in the iron lattice and form carbides,nitrides and carbonitrides with carbon and nitrogen.

Vanadium V and niobium Nb: these act in a grain-refining manner inparticular by forming carbides, whereby at the same time the strength,toughness and expansion properties are improved. Contents of more than1.5 wt. % do not provide any further advantages. Optionally, forvanadium and niobium, a minimum content of greater than or equal to0.005 wt. % and a maximum content of 0.6 (V) or 0.4 (Nb) wt. % ispreferably provided, in which the alloy elements advantageously providegrain refinement. Furthermore, in order to improve the economicfeasibility whilst at the same time achieving optimum grain refinement,the contents of V can be restricted to 0.01 wt. % to 0.3 wt. % and thecontents of Nb can be restricted to 0.01 to 0.1 wt. %.

Tantalum Ta: tantalum acts in a similar manner to niobium as a carbideforming agent in a grain-refining manner and thereby improves thestrength, toughness and expansion properties at the same time. Contentsover 0.5 wt. % do not provide any further improvement in the properties.Thus, a maximum content is optionally set to 0.5 wt. %. Preferably, aminimum content of 0.005 and a maximum content of 0.3 wt. % are set, inwhich the grain refinement can advantageously be produced. In order toimprove economic feasibility and to optimise grain refinement, a contentof 0.01 wt. % to 0.1 wt. % is particularly preferably sought.

Titanium Ti: acts in a grain-refining manner as a carbide forming agent,whereby at the same time the strength, toughness and expansionproperties are improved and the inter-crystalline corrosion is reduced.Contents of Ti of more than 1.5 wt. % impair the expansion properties,for which reason a maximum content of Ti of 1.5 wt. % is set.Optionally, a minimum content of 0.005 and a maximum content of 0.6 wt.% are set, in which Ti is advantageously precipitated. Preferably, aminimum content of 0.01 wt. % and a maximum content of 0.3 wt. % areprovided which ensure optimum precipitation behaviour with low alloycosts.

Tin Sn: tin increases the strength but, similar to copper, accumulatesbeneath the scale layer and at the grain boundaries at highertemperatures. This results, owing to the penetration into the grainboundaries, in the formation of low melting point phases and, associatedtherewith, in cracks in the microstructure and in solder brittleness,for which reason a maximum content of less than or equal to 0.5 wt. % isoptionally provided. For the aforementioned reasons, contents of lessthan 0.2 wt. % are preferably adjusted. Contents of <0.05 wt. % areparticularly advantageously preferred in order to avoid low meltingpoint phases and cracks in the microstructure.

Copper Cu: reduces the rate of corrosion and increases the strength.Contents of 3 wt. % and more impair the producibility by forming lowmelting point phases during casting and hot-rolling, for which reason amaximum content of 3 wt. % is set. Optionally, a maximum content of lessthan 0.5 wt. % is set, in which the occurrence of cracks during castingand hot-rolling can be advantageously prevented. Cu contents of <0.1 wt.% have turned out to be particularly advantageous in order to avoid lowmelting point phases and to avoid cracks.

Tungsten W: acts as a carbide forming agent and increases the strengthand heat resistance. Contents of W of more than 5 wt. % impair theexpansion properties, for which reason a maximum content of 5 wt. % W isset. Optionally, a maximum content of 3 wt. % and a minimum content of0.01 wt. % are set, in which the precipitation of carbidesadvantageously takes place. In particular, a minimum content of 0.2 wt.% and a maximum content of 1.5 wt. % are preferably provided whichpermits optimum precipitation behaviour with low alloy costs.

Cobalt Co: increases the strength of the steel, stabilises the austeniteand improves the heat resistance. Contents of more than 8 wt. % impairthe expansion properties, for which reason a maximum content of 8 wt. %is set. Optionally, a maximum content of less than or equal to 5 wt. %and a minimum content of 0.01 wt. % are set which advantageously improvethe strength and heat resistance. Preferably, a minimum content of 0.3wt. % and a maximum content of 2 wt. % are provided which advantageouslyinfluences the austenite stability along with the strength properties.

Zirconium Zr: acts as a carbide forming agent and improves the strength.Contents of Zr of more than 0.5 wt. % impair the expansion propertiesfor which reason a maximum content of 0.5 wt. % is set. Optionally, amaximum content of 0.3 wt. % and a minimum content of 0.005 wt. % areset, in which carbides are advantageously precipitated. Preferably, aminimum content of 0.01 wt. % and a maximum content of 0.2 wt. % areprovided which advantageously permit optimum carbide precipitation withlow alloy costs.

Boron B: delays the austenite conversion, improves the hot-formingproperties of steels and increases the strength at room temperature. Itachieves its effect even with very low alloy contents. Contents above0.15 wt. % greatly impair the expansion and toughness properties, forwhich reason the maximum content is set to 0.15 wt. %. Optionally, aminimum content of 0.001 and a maximum content of 0.08 wt. % are set, inwhich the strength-increasing effect of boron is advantageously used.Furthermore, a minimum content of 0.002 wt. % and a maximum content of0.01 wt. % are preferred which permit optimum use for increasingstrength whilst at the same time improving the conversion behaviour.

Tellurium Te: improves the corrosion resistance and the mechanicalproperties as well as the machining capability. Furthermore, Teincreases the strength of MnS which as a result is lengthened to alesser extent in the rolling direction during hot-rolling andcold-rolling. Contents above 0.5 wt. % impair the expansion andtoughness properties, for which reason a maximum content of 0.5 wt. % isset. Optionally, a minimum content of 0.005 wt. % and a maximum contentof 0.3 wt. % are set which advantageously improve mechanical propertiesand increase the strength of MnS present. Furthermore, a minimum contentof 0.01 wt. % and a maximum content of 0.1 wt. % are preferred whichpermit optimisation of the mechanical properties whilst at the same timereducing the alloy costs.

Calcium Ca: is used for modifying non-metallic oxidic inclusions whichcould otherwise result in the undesired failure of the alloy as a resultof inclusions in the microstructure which act as stress concentrationpoints and weaken the metal composite. Furthermore, Ca improves thehomogeneity of the alloy in accordance with the invention. In order toachieve a corresponding effect, a minimum content of 0.0005 wt. % isoptionally necessary. Contents of above 0.1 wt. % Ca do not provide anyfurther advantage in the modification of inclusions, impairproducibility and should be avoided by reason of the high vapourpressure of Ca in steel melts. Therefore, a maximum content of 0.1 wt. %is provided

Typical applications of components which are hot-formed in accordancewith the invention and are produced from metal sheets or pipes aspre-products concern in particular automotive engineering but e.g. alsomobile crane construction and longitudinal and transverse beams incommercial vehicles and trailers or safety and chassis parts inpassenger cars and wagon construction.

For example, a sheet metal plate or a pipe can be used as thepre-product. The sheet metal plate can be manufactured from a hot stripor cold strip and the pipe can be a seamlessly hot-rolled pipe or awelded pipe produced from a hot strip or cold strip.

The hot-rolled or welded pipe can be warm-formed after production onceagain with one or multiple drawing and/or annealing processes or in ahydraulic expanding process, e.g. by means of internal high pressureforming (IHU).

Moreover, in accordance with the invention it is advantageously possibleto perform the individual forming steps at different speeds and atdifferent temperatures within the temperature range in accordance withthe invention. For instance, it is possible e.g. to advantageouslyprevent the martensitic formation in the first steps in order to improvethe forming properties and facilitate further forming, and in the lastforming step to select a temperature range which permits partialmartensitic conversion of the microstructure with the aim of increasingstrength. Furthermore, it is advantageously possible to perform severalforming procedures with fewer intermediate heating procedures and thusin one extended temperature range, whereby the number of intermediateheating procedures can advantageously be reduced. Different formingspeeds similarly permit targeted influencing of the martensiticconversion and stress distribution in the component.

Furthermore, in accordance with the invention a multi-stage method canadvantageously also be performed, in which the semi-hot-forming processis followed by a final cold-forming procedure (e.g. rolling, pressing,deep-drawing, incremental forming), whereby an overall higher formingcapacity can be achieved in comparison with cold-forming alone.

The pre-product and the component produced therefrom are characterisedby very high tensile strength with sufficiently high expansion.Moreover, an effective welding capability is provided by reason of thechemical composition.

Furthermore, the pre-product can be provided in a known manner with alacquer-based scaling-inhibiting or corrosion-inhibiting layer or with ametallic coat. The metallic coat can contain zinc and/or magnesiumand/or aluminium and/or silicon. A pipe as a pre-product can be coatedboth on the inner side and outer side,

In contrast to well-established manufacturing routes, even asurface-coated hot strip or cold strip or pipe can be used for formingfollowing on from heating because semi-hot-forming sustains adhesion andductility. The metallic coat is resistant to short-term reheating of thesubstrate/coating (steel strip/coating) combination below the Ac₃temperature of the substrate in order to withstand the reheating priorto semi-hot-forming and the actual semi-hot-forming.

By reason of the comparatively small amount of heat, large-scalereheating units, such as e.g. on tunnel furnaces or chamber furnaces,can be dispensed with in favour of rapid-acting and direct-actingsystems (inductive, conductive, direct in the tool and in particularradiation).

Moreover, for the described novel method considerably less heat energyis required, or the energy efficiency is higher than in the case ofpress-hardening. As a result, the process costs are lower and the CO₂emission is reduced. In contrast to press-hardenable steels, technicalaccelerated cooling in the tool can advantageously be dispensed withdepending upon the application, thus significantly increasing thethroughput of semi-finished products per forming tool. Any technicallyaccelerated cooling which is possibly required can be performed outsidethe tool.

Preferably, reheating is performed before semi-hot-forming by means ofinduction because in this case the energy efficiency is high and heatingduration is short. Furthermore, heating can be advantageously performedby means of radiation because in this case the efficiency is similarlyconsiderably higher than heating in a furnace or with conductive heatingand energy is input into the material more rapidly and effectivelydepending upon the surface characteristics.

The material is also very suitable for partial heating. By using e.g.radiators, individual regions of the pre-product to be formed can beheated in a targeted manner in order to obtain formability-optimisedzones and to adjust the strength locally by the proportion of martensiteconverted by the TRIP effect. This advantageously permits the use ofconventional presses for cold-forming so that a complex hot-forminginstallation, as required for press-hardening, can be dispensed with.

A steel strip for producing a pre-product (strip, sheet, pipe) can beproduced from the inventive steel according to the following methodsteps:

smelting a steel melt containing (in wt. %): C: 0.0005 to 0.9 Mn: morethan 3.0 to 12, with the remainder being iron including unavoidablesteel-associated elements, with optional addition by alloying of one ormore of the following elements (in wt. %): Al: to 10, Si: to 6, Cr: to6, Nb: to 1.5, V: to 1.5, Ti: to 1.5, Mo: to 3, Cu: to 3, Sn: to 0.5, Wto 5, Co: to 8, Zr: to 0.5, Ta: to 0.5, Te: to 0.5, B: to 0.15, P: max.0.1, 5: max. 0.1, N: max. 0.1, Ca to 0.1.

casting the steel melt to form a pre-strip by means of a horizontal orvertical strip casting process approximating the final dimensions orcasting the steel melt to form a slab or thin slab by means of ahorizontal or vertical slab or thin slab casting process,

re-heating the slab or thin slab to 1050° C. to 1250° C. and thenhot-rolling the slab or thin slab to form a hot strip or thick plate, orre-heating the produced pre-strip which approximates the finaldimensions, in particular with a thickness greater than 3 mm, to 1000°C. to 1.200° C. and then hot-rolling the pre-strip to form a hot stripor thick plate, or hot-rolling the pre-strip without re-heating from thecasting heat to form a hot strip or thick plate with optionalintermediate heating between individual rolling passes of thehot-rolling,

reeling the hot strip and optionally the thick plate at a reelingtemperature between 780° C. and room temperature,

optionally annealing the hot strip or thick plate with the followingparameters: annealing temperature: 450 to 900° C., annealing duration: 1minute to 48 hours,

optionally cold-rolling the hot strip or the produced pre-strip whichapproximates the final dimensions, with a thickness of less than 5 mm toform a cold strip,

optionally annealing the cold strip with the following parameters:annealing temperature: 450 to 900° C., annealing duration: 1 minute to48 hours, a flat steel product having a good combination of strength,expansion and deformation properties, and an increased resistance todelayed crack formation and hydrogen embrittlement which has a TRIPand/or TWIP effect during mechanical loading owing to its residualaustenite content in the microstructure.

What is claimed is: 1.-38. (canceled)
 39. A method for producing acomponent with a minimum tensile strength of 700 MPa and high elongationat fracture A80 in %, said method comprising: heating a pre-product ofsteel to a temperature in a temperature range above 60° C. and below450° C., with the pre-product having a following steel composition inwt. %: C: 0.0005 to 0.9 Mn: more than 3.0 to 12, with the remainderbeing iron including unavoidable steel-associated elements; andhot-forming the pre-product in said temperature range, wherein aresidual austenite proportion is 5% to 80%.
 40. The method of claim 39,wherein the pre-product includes at least one alloying element selectedfrom the group consisting of (in wt. %): Al: up to 10 Si: up to 6 Cr: upto 6 Nb: up to 1.5 V: up to 1.5 Ti: up to 1.5 Mo: up to 3 Cu: up to 3Sn: up to 0.5 W: up to 5 Co: up to 8 Zr: up to 0.5 Ta: up to 0.5 Te: upto 0.5 B: up to 0.15 P: max. 0.1, in particular <0.04 S: max. 0.1, inparticular <0.02 N: max. 0.1, in particular <0.05 Ca: up to 0.1.
 41. Themethod of claim 39, wherein the steel contains (in wt. %) C: 0.05 to0.42.
 42. The method of claim 39, wherein the steel contains (in wt. %)Mn: >5 to <10.
 43. The method of claim 39, wherein the steel contains(in wt. %) Al: 0.1 to 5, in particular >0.5 to
 3. 44. The method ofclaim 39, wherein the steel contains (in wt. %) Si: 0.05 to 3, inparticular >0.1 to 1.5.
 45. The method of claim 39, wherein the steelcontains (in wt. %) Cr: 0.1 to 4, in particular >0.5 to 2.5.
 46. Themethod of claim 39, wherein the steel contains (in wt. %) Nb: 0.005 to0.4, in particular 0.01 to 0.1.
 47. The method of claim 39, wherein thesteel contains (in wt. %) V: 0.005 to 0.6, in particular 0.01 to 0.3.48. The method of claim 39, wherein the steel contains (in wt. %) Ti:0.005 to 0.6, in particular 0.01 to 0.3.
 49. The method of claim 39,wherein the steel contains (in wt. %) Mo: 0.005 to 1.5, in particular0.01 to 0.6.
 50. The method of claim 39, wherein the steel contains (inwt. %) Sn: <0.2, in particular <0.05.
 51. The method of claim 39,wherein the steel contains (in wt. %) Cu: <0.5, in particular <0.1. 52.The method of claim 39, wherein the steel contains (in wt. %) W: 0.01 to3, in particular 0.2 to 1.5.
 53. The method of claim 39, wherein thesteel contains (in wt. %) Co: 0.01 to 5, in particular 0.3 to
 2. 54. Themethod of claim 39, wherein the steel contains (in wt. %) Zr: 0.005 to0.3, in particular 0.01 to 0.2.
 55. The method of claim 39, wherein thesteel contains (in wt. %) Ta: 0.005 to 0.3, in particular 0.01 to 0.1.56. The method of claim 39, wherein the steel contains (in wt. %) Te:0.005 to 0.3, in particular 0.01 to 0.1.
 57. The method of claim 39,wherein the steel contains (in wt. %) B: 0.001 to 0.08, in particular0.002 to 0.01.
 58. The method of claim 39, wherein the steel contains(in wt. %) Ca: 0.005 to 0.1.
 59. The method of claim 39, wherein thepre-product is heated only partially at a temperature range of above 60°C. to below 450° C.
 60. The method of claim 39, wherein the pre-productis heated to a temperature of below 700° C.
 61. The method of claim 39,wherein the temperature range in which the pre-product is heated rangesfrom 450 to below 700° C.
 62. The method of claim 39, wherein thetemperature range in which the pre-product is heated ranges from 350 tobelow 450° C.
 63. The method of claim 39, wherein the temperature rangein which the pre-product is heated ranges from 60 to below 350° C. 64.The method of claim 39, further comprising applying a metallic orlacquer-like coat on the pre-product prior to the pre-product beingheated.
 65. The method of claim 64, wherein the metallic coat containsat least one element selected from the group consisting of Zn, Mg, Aland Si.
 66. The method of claim 64, wherein the metallic coat contains aZn alloy selected from the group consisting of ZnMg, ZnAl, ZnNi, ZnFe,ZnCo or ZnAlCe.
 67. The method of claim 39, wherein the pre-product isheated to the temperature inductively, conductively, by radiation, or byheat conduction in a forming tool and further comprising cooling thepre-product, after forming, in air or in a technically acceleratedmanner by means of moving gases, air or liquid media in or outside theforming tool.
 68. The method of claim 39, wherein the pre-product is asheet metal plate or a pipe.
 69. The method of claim 68, wherein thesheet metal plate is made of a hot strip or cold strip.
 70. The methodof claim 68, wherein the pipe is a seamlessly hot-rolled pipe or awelded pipe which is produced from hot strip or cold strip, and furthercomprising coating the pipe with an inner coating and/or outer coating.71. The method of claim 68, wherein the pipe is a seamlessly hot-rolledpipe or a welded pipe which is produced from a hot strip or cold stripand which, in the course of the hot-forming procedure, is subjected toone or multiple drawing and/or annealing processes.
 72. The method ofclaim 39, further comprising subjecting the pre-product afterhot-forming to final cold-forming.
 73. A hot-formed component, producedfrom a steel comprising an alloy composition in wt. %: C: 0.0005 to 0.9Mn: more than 3.0 to 12 with the remainder being iron includingunavoidable steel-associated elements, with optional addition byalloying of one or more of the following elements (in wt. %): Al: up to10 Si: up to 6 Cr: up to 6 Nb: up to 1.5 V: up to 1.5 Ti: up to 1.5 Mo:up to 3 Cu: up to 3 Sn: up to 0.5 W: up to 5 Co: up to 8 Zr: up to 0.5Ta: up to 0.5 Te: up to 0.5 B: up to 0.15 P: max. 0.1, in particular<0.04 S: max. 0.1, in particular <0.02 N: max. 0.1, in particular <0.05Ca: up to 0.1. by hot-forming a pre-product of said steel, in which thepre-product is heated to a temperature of 60° C. to below the Ac₃conversion temperature and is then formed, wherein the component has aminimum tensile strength of 700 MPa to over 1350 MPa with asimultaneously high elongation at fracture A80 and the product oftensile strength x elongation at fracture has at least the followingvalues: Rm of 700 to 800 MPa: Rm×A80≥15400 up to 50000 Rm of over 800 to900 MPa: Rm×A80≥14400 up to 50000 Rm of over 900 to 1100 MPa:Rm×A80≥13500 up to 45000 Rm of over 1100 to 1200 MPa: Rm×A80≥13200 up to45000 Rm of over 1200 to 1350 MPa: Rm×A80≥11200 up to 45000 Rm of over1350 to 1800 MPa: Rm×A80≥8000 up to 45000 Rm of over 1800 MPa:Rm×A80≥4000 up to
 30000. 74. The hot-formed component of claim 73,comprising an alloy composition in wt. %: C: 0.05 to 0.42 Mn: >5 to <10with the remainder being iron including unavoidable steel-associatedelements, with optional addition by alloying of one or more of thefollowing elements (in wt. %): Al: 0.1 to 5, in particular >0.5 to 3 Si:0.05 to 3, in particular >0.1 to 1.5 Cr: 0.1 to 4, in particular >0.5 to2.5 Nb: 0.005 to 0.4, in particular 0.01 to 0.1 B: 0.001 to 0.08, inparticular 0.002 to 0.01 Ti: 0.005 to 0.6, in particular 0.01 to 0.3 Mo:0.005 to 1.5, in particular 0.01 to 0.6 Sn: <0.2, in particular <0.05Cu: <0.5, in particular <0.1 W: 0.01 to 3, in particular 0.2 to 1.5 Co:0.01 to 5, in particular 0.3 to 2 Zr: 0.005 to 0.3, in particular 0.01to 0.2 Ta: 0.005 to 0.3, in particular 0.01 to 0.1 Te: 0.005 to 0.3, inparticular 0.01 to 0.1 V: 0.005 to 0.6, in particular 0.01 to 0.3 Ca:0.0005 to 0.1.
 75. The hot-formed component of claim 73, produced by amethod as set forth in claim
 59. 76. The hot-formed component of claim75, for use in the automotive and commercial vehicle industries, inengineering, construction or for producing white goods.